The organic EL device is a self-emitting device, and has been actively studied for their brighter, superior viewability and ability to display clearer images compared with the liquid crystal device.
In 1987, C. W. Tang et al. at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic EL device with organic materials. These researchers laminated tris(8-hydroxyquinoline)aluminum (an electron-transporting phosphor; hereinafter, simply Alq3), and a hole-transporting aromatic amine compound, and injected the both charges into the phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (see, for example, Patent Documents 1 and 2).
To date, various improvements have been made for practical applications of the organic EL device. In order to realize high efficiency and durability, various roles are further subdivided to provide an electroluminescent device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode successively formed on a substrate (see, for example, Non-Patent Document 1).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and use of phosphorescent materials have been investigated (see, for example, Non-Patent Document 2).
The light emitting layer can also be fabricated by doping a charge-transporting compound, generally called a host material, with a phosphor or a phosphorescent material. As described in the foregoing lecture preprints, selection of organic materials in an organic EL device greatly influences various device characteristics, including efficiency and durability.
In an organic EL device, the charges injected from the both electrodes recombine at the light emitting layer to cause emission. The probability of hole-electron recombination can be improved by improving the hole injectability and the electron blocking performance of blocking the injected electrons from the cathode, and high luminous efficiency can be obtained by confining the excitons generated in the light emitting layer. The role of the hole transport material is therefore important, and there is a need for a hole transport material that has high hole injectability, high hole mobility, high electron blocking performance, and high durability to electrons.
The aromatic amine derivatives described in Patent Documents 1 and 2 are known examples of the hole transport materials used for the organic EL device. These compounds include a compound known to have an excellent hole mobility of 10−3 cm2/Vs or higher. However, the compound is insufficient in terms of electron blocking performance, and some of the electrons pass through the light emitting layer. Accordingly, improvements in luminous efficiency cannot be expected.
Arylamine compounds of the following formulae having a substituted carbazole structure (for example, Compounds A, B, and C) are proposed as improvements over the foregoing compounds (see, for example, Patent Documents 3 to 5).

In an attempt to improve the device luminous efficiency, there have been developed devices that use phosphorescent materials to generate phosphorescence, specifically that make use of the emission from the triplet excitation state. According to the excitation state theory, phosphorescent materials are expected to greatly increase luminous efficiency about four times as much as that of the conventional fluorescence.
In 1999, M. A. Baldo et al. at Princeton University realized 8% luminous efficiency with a phosphorescent device using an iridium complex, a great improvement over the conventional external quantum efficiency. The phosphorescent device has been actively developed ever since.
Improving the luminous efficiency of the phosphorescent device requires use of materials of high excitation triplet energy level (hereinafter, simply “T1”) for the host material. However, there is a report that use of materials with high T1 is also necessary for the hole transport material to confine the triplet excitons (see, for example, Non-Patent Document 3). Further, the green phosphorescent material tris(phenylpyridyl)iridium (hereinafter, simply “Ir(ppy)3”) represented by the following formula has a T1 of 2.42 eV.

Because N,N′-diphenyl-N,N′-di(α-naphthyl)benzidine (hereinafter, simply “α-NPD”) has a T1 of 2.29 eV, sufficient confinement of the triplet excitons cannot be expected with α-NPD. Higher luminous efficiency is thus obtained using 1,1-bis[4-(di-4-tolylamino)phenyl]cyclohexane (hereinafter, simply “TAPC”) of the following formula having a higher T1 value of 2.9 eV (see, for example, Non-Patent Document 4).

However, the TAPC has low hole mobility, and its ionization potential (work function) 5.8 eV is not appropriate for a hole transport material.
The ionization potential (work function) of Compound A is 5.5 eV, a more appropriate value compared to the ionization potential of the TAPC. It is expected that this, combined with the high T1 of 2.9 eV, would provide sufficient confinement of the triplet excitons. However, because the compound has low hole mobility, the product device has high driving voltage, and the luminous efficiency cannot be said as sufficient (see, for example, Non-Patent Document 5). Accordingly, there is a need for materials having a high T1 value and high hole mobility that can be used as a hole injection layer or a hole transport layer, in order to obtain a phosphorescent device having improved luminous efficiency.